The present invention relates to a vaulted tank and, more particularly, to an above-ground insulated and sealed storage tank for flammable liquids.
Since the 1970s, the world, and in particular, the United States, has been concerned with the environment and the contamination of that environment, including the earth's soil, its atmosphere and its water. The first Earth Day in 1970 resulted in the eventual creation of the Environmental Protection Agency by the United States Congress.
One of the many problems which the Environmental Protection Agency has addressed is the deterioration of large, underground storage tanks and the resulting leakage of contaminants into the soil. An example of this is the well-documented and widespread deterioration of gasoline station storage tanks and the leakage of gasoline and diesel fuel into the surrounding water table.
To correct this problem, the EPA has suggested that all fuel storage tanks be placed above ground. This has created a classic confrontation between governmental departments, for the fire departments of most major cities prefer that fuel storage tanks be placed below ground to reduce fire hazard, and most municipal codes have been drafted with this concern in mind. In recent years, the creation of large concrete-entombed, above ground tanks has been suggested as a solution to the problem. That is, a gasoline storage tank is entombed in concrete and placed above the ground to enable its surfaces to be easily checked for deterioration and fluid leakage. By entombing it in concrete, the tank is made impervious to impact from a vehicle that might back into it, for example, and becomes resistant to fire due to the insulating effect of the concrete. Such insulation is designed to provide the minimum two-hour fire resistive protection required by the Uniform Fire Code and the National Fire Protection Agency for above ground tanks. One example of such an entombed tank is shown in U.S. Pat. No. 4,826,644, issued May 2, 1989 to T. R. Lindquist and R. Bambacigno.
The concrete entombed tank has several disadvantages, including its high cost and lack of convenience. For example, a 1,000-gallon concrete-entombed tank weighs 18,000 pounds after it has been manufactured. Such a tank requires a large truck and crane with at least two 20-ton nylon straps to transport it to the site where it is to be used and to then place the tank in the desired position. This is not only an arduous undertaking, but it is an expensive one.
The prior art attempted to avoid the problem of transporting such tanks by constructing the concrete casing on site. However, this required the building of special forms on site, and the transport of concrete to the site for pouring. This presented serious logistical problems where the site was in a remote location and often added to the cost.
A further disadvantage of the concrete-entombed tank is that such tanks do not have long-term structural integrity, and they have caused considerable concern in this regard. In order to cover such tanks with concrete, a relatively thick layer is required, adding to the weight of the entire device, and requiring reinforcement of the inner tank so it will withstand the pressure of the entombing material. However, even relatively thick layers of concrete present a reliability problem, for exposure of such tanks to extreme weather conditions, such as wide temperature variations, will cause concrete to crack. It has been found that in as little as a year's time, spider cracks can appear in a concrete casing for a storage tank, thereby compromising its structural integrity. This problem occurs for other casing materials, as well, for foam insulation or light-weight concrete, while reducing the weight problem, still suffer from exposure to the elements and gradually degrade.
A further problem which occurs with cement-like encasement materials is that of rapid failure in case of fire. Although such material is intended to insulate the inner tank, in fact it absorbs heat, so that when water is directed onto the concrete (or like material) it will often crack and fall away from the tank, thus exposing the tank directly to the heat.